The present invention relates to the transmission of digital data, particularly the transmission of digital data that represents video signals.
It is generally acknowledged that some form of digital transmission will be required for the next generation of television (TV) technology, conventionally referred to as high definition television, or HDTV. This requirement is due mostly to the fact that much more powerful video compression schemes can be implemented with digital signal processing than with analog signal processing. However, in any HDTV digital transmission system, there are three major areas of concern that have to be addressed: graceful degradation, NTSC (National Television System Committee) co-channel interference and ghost cancellation.
A number of co-pending, commonly assigned United States patent applications disclose various techniques that provide graceful degradation in the reception quality at a TV set location. There are: V. B. Lawrence et al. entitled "Coding for Digital Transmission," Ser. No. 07/611,225, filed on Nov. 7, 1990; L.-F. Wei entitled "Coded Modulation with Unequal Error Protection," Ser. No. 07/611,200, filed on Nov. 7, 1990; J. D. Johnston et al. entitled "A High Definition Television Coding Arrangement with Graceful Degradation," Ser. No. 07/625,349, filed on Dec. 11, 1990; and H. Y. Chung et al. entitled "Multiplexed Coded Modulation with Unequal Error Protection," Ser. No. 07/627,156, filed on Dec. 13, 1990. The Lawrence et al. patent application, for example, teaches the notion of characterizing the HDTV signal into classes of "more important" and "less important" information, which will then use a constellation of non-uniformly spaced signal points. This approach provides unequal error protection, i.e., more error protection for the more important information, and allows a graceful degradation in reception quality at the TV set location because, as the bit-error rate at the receiver begins to increase with increasing distance from the broadcast transmitter, it will be the bits that represent proportionately less of the TV signal information that will be the first affected.
However, although the above-mentioned patent applications teach advantageous techniques for providing unequal error protection to different classes of information, these approaches primarily address the problem of providing graceful degradation for an HDTV signal in a single carrier transmission environment and do not address the problems of NTSC co-channel interference and ghost cancellation.
NTSC co-channel interference is a result of the fact that any HDTV transmission scheme will co-exist with existing NTSC TV transmission schemes and will use the available NTSC frequency spectrum, or channel assignments. For example, in the New York City geographical area an HDTV television station may be assigned to broadcast on channel 3. However, there may also be an NTSC television station assigned to channel 3 in a neighboring geographical area such as Philadelphia. As a result, there will be parts of New Jersey that receive both the HDTV and NTSC television signals assigned to channel 3. This results in a geographical region of overlap of the NTSC and HDTV transmission signals in which the NTSC and HDTV signals interfere with each other. To reduce the interference from the HDTV signal to the existing NTSC signal, the transmitted power of the HDTV signal should be set at a value at least 10 dB below that of the NTSC signal so that the HDTV signal does not interfere with the NTSC signal. As a result, the HDTV signal is even more susceptible to interference from the NTSC signal. This NTSC interference must be reduced in order to ensure that the coverage area of the HDTV signal is large enough.
Finally, there is the problem of ghost cancellation. In any TV transmission scheme, reflection of the transmitted signal may occur that results in ghosting, which generally manifests itself in the form of double images. However, the problem of ghosting is compounded in an HDTV transmission scheme because of the use of compression algorithms to squeeze a full-bandwidth HDTV signal, e.g., 800M bits/sec., into an NTSC 6 MHz channel. This necessitates the use of a complex equalizer to cancel the ghost images in an HDTV transmission scheme.
Before proceeding with a description of an illustrative embodiment, it should be noted that the various digital signaling concepts described herein--with the exception, of course, of the inventive concept itself--are all well known in, for example, the digital radio and voiceband data transmission (modem) arts and thus need not be described in detail herein. These include such concepts as multidimensional signaling using 2N-dimensional channel symbol constellations, where N is some integer; trellis coding; fractional coding; scrambling; passband shaping; equalization; Viterbi, or maximum-likelihood, decoding; etc.